1. Field of the Invention
This invention relates to a method of forming a reaction product such as calcium silicate, structure employed in the method for forming this reaction product, and the resulting reaction product.
2. Prior Art
Shaped calcium silicate insulation is widely used, particularly for applications involving temperatures above 800.degree. fahrenheit. A variety of processes for forming calcium silicate insulation products are known. For example, U.S. Pat. Nos. 3,988,419, 2,699,097, 2,904,444, and 3,001,882 disclose methods for forming calcium silicate insulation. As disclosed in the '882 patent, the calcium silicate insulation typically is composed of crystals of synthetic tobermorite and/or xonotlite prepared by the induration of aqueous lime-silica slurries in which the molar ratio of lime-silica falls in the range of about 0.65:1 to 1:1 and the water-to-solids ratio falls between about 0.75:1 to about 9.0:1. Typically, in the preparation of low density insulation (i.e., densities ranging from about 5 to 15 lbs. per cubic foot) asbestos fibers have been added as a reinforcing material to the slurry. A description of certain prior art techniques for producing molded materials of calcium silicate is described in U.S. Pat. No. 3,679,446 on an application of Kubo. Kubo states that it is difficult to obtain calcium silicate insulation products with uniform properties and satisfactory mechanical strength unless the induration reaction is conducted for a long period of time. Kubo further states that the calcium silicates typically produced cannot satisfactorily withstand high temperatures with the result that a calcium silicate product composed mainly of tobermorite crystals is liable to decrease in mechanical strength markedly at 650.degree. C. or thereabout and to disintegrate or break down at over 700.degree. C. and that a product composed mainly of xonotlite crystals tends to decrease in mechanical strength markedly at a temperature higher than about 1000.degree. C. Kubo discloses a method of forming calcium silicate crystals wherein at least a given percent by weight of the calcium silicate crystals has formed therein "numerous small agglomerates of a diameter of ten to 150 microns by being three dimensionally interlocked with one another," said agglomerates being dispersed in water "in substantially globular form." Kubo also discloses the use of reinforcing fibers formed predominantly of pulp fiber.
Hoopes and Weber disclose in U.S. Pat. No. 3,736,163 the formation of calcium silicate insulating material having densities on the order of 10 to 15 lbs. per cubic foot wherein asbestos reinforcing fibers are replaced by a reinforcing fiber comprising from about three percent to fifteen percent of the weight of the calcium silicate material and consisting of nodulated mineral wool and cellulosic fiber, at least about twenty-five percent of the fiber being nodulated mineral wool.
In the manufacture of calcium silicate products in accordance with the prior art, the reaction constituents (typically calcium oxide (CaO) or hydrated calcium oxide Ca(OH).sub.2) are mixed with a siliceous material, such as sand, in water to form a slurry. This mixture is heated in an autoclave to form a variety of crystalline forms of calcium silicate depending upon the temperature, pressure, length of reaction time and water concentration used. Fibrous materials such as asbestos, which are not adversely affected by the reaction conditions, may be incorporated into the mixture prior to processing. The reaction product of this processing is generally an aqueous slurry of hydrated calcium silicate crystals intermixed with desired fibrous components. This slurry is then cast into molds and dried, usually by heating, to form the desired finished shaped objects.
As discussed in the Zettel U.S. Pat. No. 3,816,149, this processing to form the crystalline materials in the slurry is time consuming and requires large and expensive pieces of processing equipment. Thus the prior art has attempted to improve the process conditions under which crystallization takes place and to shorten the time required to produce a finished hydrate. Decreasing the time to process the slurry through crystallization and the return to ambient conditions results in more efficient and economical utilization of the equipment and an increased output of finished product.
In the prior art typically the slurry was cooled in the autoclave upon completion of the crystallization. The pressure was then reduced within the autoclave while the slurry was cooled to ambient. The prior art recognized that allowing the steam pressure in the autoclave to be reduced by cooling was both slow (because of the long time required to transfer heat from the slurry through the autoclave walls to the ambient) and inefficient (because of the waste of the heat so transferred). To speed up the process, the high pressure steam in the autoclave was vented to the atmosphere. Because at atmospheric pressure the crystalline reaction of the components in the slurry proceeds at a temperature well above the boiling point of water, the venting of steam to the atmosphere caused the hot aqueous slurry to boil violently. This fractured many of the newly formed crystals thus defeating the purpose of the careful prior crystallization. Moreover, the venting of steam wasted energy. As disclosed in U.S. Pat. No. 3,816,149, Zettel attempted to overcome these problems by hydrothermally reacting a highly concentrated aqueous slurry of a source of calcium oxide and a siliceous material in the presence of saturated steam under elevated pressure in a pressure vessel to form crystalline calcium silicate. Following the formation of the crystalline calcium silicate hydrate, the steam input was halted and low temperature water was gradually added to the reaction mixture within the pressure vessel until sufficient water was added to dilute the crystalline slurry to the desired concentration for subsequent molding operations. Incoming water condensed the steam in the pressure vessel simultaneously reducing its pressure and cooling the crystal-containing slurry. The gradual cooling and depressurization was described by Zettel as effectively eliminating the disruption of the crystal structure. Zettel also reduced substantially the time required to raise the reaction mixture from ambient to the condition for the crystallization reaction to take place by reducing the amount of water present which must be heated compared to the then prior art processes.
Calcium silicate insulation produced by the prior art still leaves much to be desired in the way of strength, high temperature insulation capability, predictability of characteristics, machineability, and dimensional consistency. In addition, the manufacturing process for this material still wastes considerable energy by heating and cooling the reaction slurry in the autoclave. This process also increases the cost of forming insulation by tying up the equipment for a long period of time per batch of calcium silicate formed. Finally, during the transfer of the calcium silicate reaction product from the autoclave to a holding tank for the next stage in the operation, the crystalline structure of the reaction product fractures or is otherwise changed.